The present invention relates to a motor drive unit such as a light-amount adjusting apparatus or a focusing apparatus used for an image-pickup optical device such as a video camera or a digital camera, and to an optical apparatus having the motor drive unit.
A camera such as a video camera or a digital camera having a solid-state image pickup device built-in, or a camera using film has a stop apparatus (a light-amount adjusting apparatus) which controls an aperture diameter in order to confirm a focal depth of a lens, and to adjust a light amount of an object imaged on a film or a solid-state image pickup device.
Such a camera has, not only a stop apparatus, but also a focusing apparatus which automatically adjusts a focal point of a shooting lens.
Some optical apparatuses for projecting images also have a light-amount adjusting apparatus or a focusing apparatus therein.
These light-amount adjusting apparatuses are mainly configured so as to vary their aperture diameters by driving a plurality of stop blades serving as light-shielding members with stepper motors serving as sources of driving force.
Further, those focusing apparatuses are mainly configured so as to adjust a focal point by driving some of shooting lens in its optical axis direction with stepper motors serving as sources of driving force.
In recent years, it has been desired to shorten a shutter time-lag in still image shooting by a camera, which has brought about an important issue that shortens a time required for stop driving or lens driving by driving a stepper motor to a target position at a high speed.
However, because a stepper motor may have a step-out due to high-speed driving or load fluctuation, it is necessary to set limits to a driving speed or calculate on a safety factor in torque. Therefore, the original performance cannot be used to the full.
In order to perform high-speed driving by using a stepper motor, a method of using a motor having wide steps or a method for reducing a reduction ratio of a transmission mechanism is conceivable. However, in this case, it is difficult to obtain a high resolution, which deteriorates accuracy in an aperture diameter or accuracy in focusing.
In a light-amount adjusting apparatus or a focusing apparatus with a stepper motor serving as a source of driving force, in a case of a digital camera for example, the apparatus starts energization of the stepper motor from a predetermined phase to determine an aperture diameter or a lens stop position in accordance with how many steps the rotor will be made to rotate.
Therefore, in order to obtain a high-accuracy aperture diameter or lens stop position, an angle of rotating the rotor is preferably smaller.
Further, in order to speed up a stepper motor without growing the stepper motor in size, a method for increasing a voltage or an electric current energizing in the motor may be used. In this case rapidly exhausts a battery serving as a power source.
Then, in order to avoid the battery serving as a power source from being consumed, after driving a motor to a predetermined position, it is preferable to turn off the energization of the motor at a high ratio. To that end, it is preferable to have many stop positions at which a rotor is capable of stably stopping per rotation of the rotor by cogging torque.
A light-amount adjusting apparatus or a focusing apparatus with a stepper motor serving as a source of driving force generally uses 1-2 phase excitation driving.
However, a number of positions at which the rotor is capable of stably stopping by cogging torque when turning off energization is the half of a number of positions at which the rotor is capable of stopping when continuing energization.
Turning off energization at a stop position at which the rotor becomes unsteady when turning off energization makes it impossible to specify whether the rotor shifts in a forward direction or shifts in the backward direction. As a result, an error of ± one step is brought about, which does not provide high accuracy in stopping when turning off energization.
Therefore, also when exposing for a relatively long time, it is necessary to continue energization of a coil in order to obtain high accuracy in stopping, which rapidly exhausts a battery serving as a power source.
As a solution to this problem, Japanese Patent No. 02566031 discloses a stepper motor which is configured so as to provide grooves at positions shifted by a predetermined angle from the center of the magnet pole on the outer circumferential surface of the magnet, to be capable of specifying a direction of rotating by cogging torque when turning off energization, which enables an improvement in stop accuracy without exhausting the battery during a long time exposure.
Moreover, Japanese Patent Laid-Open No. 10-282395 discloses a pulse motor by 1-2 phase excitation driving which is capable of obtaining required accuracy while lowering power consumption.
This motor using 1-2 phase excitation driving is configured such that, in a case of high-accuracy driving, the energization is turned off when stopping at a 1 phase excitation position, and the energization is maintained when stopping at a 2 phase excitation position. In a case of usual driving, the motor stops only at a 1 phase excitation position and the energization is turned off.
However, in the electromagnetic-driven light exposure amount adjusting apparatus disclosed in Japanese Patent No. 02566031, it is necessary to provide grooves in the outer circumferential surface of the magnet, and a sintered magnet or a compression magnet with strong magnetic force cannot be formed into such an accurate complicated shape.
Therefore, Japanese Patent No. 02566031 has a limitation that it is necessary to use an injection magnet with weak magnetic force.
Therefore, the motor has low torque or is required to increase a magnet diameter or a number of coil turns, which leads to a necessity to grow the motor in size.
Further, the outer circumferential surface of the magnet has grooves, which increases cogging torque. Thus, it is necessary to further grow the coil in size in order to operate the motor at a lower voltage and loud driving noise is brought about.
The pulse motor drive unit for a camera disclosed in Japanese Patent Laid-Open No. 10-282395, the energization is always to be maintained at a 2 phase excitation position in a case of high-accuracy driving. In a case of usual driving, the motor only stops at a 1 phase excitation position.